1. Field of the Invention
The present invention relates to high-power light-emitting devices with very low series resistance, improved current spreading and improved heat-sinking.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Photonic semiconductor devices fall into three categories: (1) devices that convert electrical energy into optical radiation (e.g., light emitting diodes and laser diodes); (2) devices that detect optical signals (e.g., photodetectors); and (3) devices that convert optical radiation into electrical energy (e.g., photovoltaic devices and solar cells). Although all three kinds of devices have useful applications, the light-emitting device may be the most commonly recognized because of its application to various consumer products and applications
A light-emitting device is a widely used semiconductor device whose main characteristic is that it emits energy in the form of light when a current flows through the device. The basic mechanisms by which light-emitting devices operate are well understood in this art and are set forth, for example, by Sze, Physics of Semiconductor Devices, 2d Edition (1981) at pages 681-703, which is incorporated by reference herein.
The wavelength of light (i.e., its color) that can be emitted by a given material of the light-emitting device is limited by the physical characteristics of that material, specifically its bandgap energy. Bandgap energy is the amount of energy that separates a lower-energy valence band and a higher energy conduction band in a semiconductor. The bands are energy states in which carriers (i.e., electrons or holes) can reside in accordance with well-known principles of quantum mechanics. The “band gap” is a range of energies between the conduction and valence bands that are forbidden to the carriers (i.e., the carriers cannot exist in these energy states). Under certain circumstances, when electrons and holes cross the bandgap and recombine, they will emit energy in the form of light. In other words, the frequency of electromagnetic radiation (i.e., the color) that can be produced by a given semiconductor material is a function of that material's bandgap energy. The full spectrum of wavelength can be covered by semiconductors.
A light-emitting device typically includes a diode structure (i.e., a p-n junction) with an active region to improve confinement of electron and hole wavefunctions for a better recombination efficiency. The active region can comprise a single quantum well and multiple quantum wells, a p-n homojunction, single heterojunction or double heterojunction, with a single or multiple quantum well structure being the most preferred. The nature of quantum wells and their advantages are generally well understood in this art as are the nature and advantages of heterojunctions with respect to homojunctions and vice versa. Accordingly, these will not be described in detail herein other than as necessary to describe the invention.
High brightness light-emitting devices involve high drive-current operation. However, large series-resistance, because of the contacts and the bulk p-regions and n-regions of the devices, results in heating. This leads to the saturation of the output power at higher continuous-wave (CW) drive-currents.
Moreover, high brightness light-emitting devices generally require very high output power. However, conventional light emitters employing semiconductor have a large turn-on voltage as well as a large series resistance because of the resistivity of the bulk p-layers and n-layers. This prevents high current operation because the device generates huge amount of heat with high input electrical power. In addition, the heat generated at higher drive currents leads to the roll-off of the output power.
What is needed, then, is a design for improved high brightness light-emitting devices. Specifically, what is needed is a light-emitting device with very low series-resistance and improved heat-sinking, effectively rendering high current operation of the device for high power light emitting. The present invention satisfies these needs via the development of a new mask design to minimize the effective series resistance, improve current spreading and also allowing better heat sinking.